Filas de consulta para las que no existe la clave de metadatos

Usa un externo unirse a través de una left join y tomar filas que no unirse:

SELECT DISTINCT wposts.ID AS ID
FROM `doxy_posts` AS wposts
left JOIN `doxy_postmeta` AS postmeta ON wposts.ID = postmeta.post_id
WHERE postmeta.post_id is null
ORDER BY wposts.post_date DESC